1. Field of the Invention
The present invention relates to a fuel cell stack comprising a plurality of fuel cell units each composed of a solid polymer ion exchange membrane interposed between an anode electrode and a cathode electrode, the plurality of fuel cell units being stacked in the horizontal direction with separators intervening therebetween.
2. Description of the Related Art
For example, the solid polymer type fuel cell comprises a fuel cell unit including an anode electrode and a cathode electrode disposed opposingly on both sides of an ion exchange membrane composed of a polymer ion exchange membrane (cation exchange membrane) respectively, the fuel cell unit being interposed between separators. Usually, the solid polymer type fuel cell is used as a fuel cell stack comprising a predetermined number of the fuel cell units and a predetermined number of the separators which are stacked with each other.
In such a fuel cell stack, a fuel gas such as a hydrogen-containing gas, which is supplied to the anode electrode, is converted into hydrogen ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the ion exchange membrane which is appropriately humidified. The electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas such as a gas containing oxygen or air is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen gas are reacted with each other on the cathode electrode, and thus water is produced.
When the fuel cell stack as described above is intended to be carried on a vehicle or the like, it is necessary that each of the fuel cell units is designed to have a large power generation area in order to obtain desired electric power. As a result, the entire fuel cell stack has a considerably large size. However, the appropriate place to accommodate the fuel cell stack for the vehicle is under the floor. It is desirable that the vehicle-carried type fuel cell stack is constructed to have a rectangular configuration with a horizontal length longer than a vertical length in which the dimension in the height direction is designed to be low. In view of this fact, for example, as disclosed in U.S. Pat. No. 5,804,326, a fuel cell stack is known, in which fuel cell units each having a rectangular configuration are constructed, and the plurality of fuel cell units are stacked by being interposed between separators.
However, in the case of the conventional technique described above, a reaction gas flow passage and a cooling medium flow passage are provided on an identical surface of the separator. The cooling medium flow passage interposes the reaction gas flow passage, and it extends linearly in the direction of the long side. For this reason, it is impossible to supply the cooling water to the entire power generation surface. It is feared that the power generation surface cannot be cooled efficiently.
Further, the cooling medium flow passage extends in the longitudinal direction of the rectangular separator. As a result, the following problem is pointed out. That is, the cooling medium flow passage is lengthy, the large pressure loss is generated, and the temperature distribution arises in the separator surface.
On the other hand, it is conceived that a single fuel cell stack is constructed by stacking a considerably large number of fuel cell units in order to obtain desired electric power. However, the following inconvenience arises. That is, the length of the fuel cell stack in the stacking direction is considerably lengthy, and it is impossible to uniformly supply the fuel gas to the respective fuel cell units.
In view of the above, a fuel cell system is adopted, which is constructed by preparing a plurality of fuel cell stacks and connecting the respective fuel cell stacks in series by the aid of a manifold. For example, in Japanese Laid-Open Patent Publication No. 8-171926, four stacks (fuel cell stacks) are prepared. Two of the stacks, which are arranged in two rows in the stacking direction respectively, are arranged in series by installing a supply/discharge member for the fuel and the like. The supply/discharge member for the fuel and the like is provided, at mutually opposing vertical surfaces at both ends, with holes for supplying/discharging the fuel and the like with respect to the two stacks respectively. Further, the supply/discharge member for the fuel and the like is formed with flow passages for making communication between the respective holes at the inside of the supply/discharge member for the fuel and the like.
In the conventional technique described above, the respective two stacks are juxtaposed and arranged on the both end surfaces of the supply/discharge member for the fuel and the like. A pressurizing mechanism is arranged on an end surface disposed on a side opposite to the supply/discharge member for the fuel and the like of each of the stacks so that the stacks are pressurized in the stacking direction. Further, an upper case and a lower case are installed to upper and lower portions of the stack. Therefore, the following problem is pointed out. That is, the assembling operation for the entire fuel cell is complicated, and the arrangement of the supply/discharge member for the fuel and the like is considerably complicated. The supply/discharge member for the fuel and the like has a large size and a complicated structure, and the production cost is expensive.
A general object of the present invention is to provide a fuel cell stack which makes it possible to suppress the dimension in the height direction of an entire fuel cell stack to be low, and cool the power generation surface uniformly and smoothly.
A principal object of the present invention is to provide a fuel cell system comprising two fuel cell stacks arranged in parallel, which makes it possible to simplify the piping structure for supplying and discharging fluids such as a fuel gas to the respective fuel cell stacks, and effectively suppress the dimension in the height direction of the fuel cell stack to be low.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.